Vehicle emission control systems may be configured to store fuel vapors from fuel tank refueling and diurnal engine operations in a fuel vapor canister, and then purge the stored vapors during a subsequent engine operation. The stored vapors may be routed to engine intake for combustion, further improving fuel economy.
In a typical canister purge operation, a canister purge valve coupled between the engine intake and the fuel canister is opened, allowing for intake manifold vacuum to be applied to the fuel vapor canister. Simultaneously, a canister vent valve coupled in a vent line between the fuel vapor canister and atmosphere is opened, allowing for fresh air to enter the canister. This configuration facilitates desorption of stored fuel vapors from the adsorbent material in the canister, regenerating the adsorbent material for further fuel vapor adsorption.
However, the vent line is prone to becoming blocked or clogged over time, as dirt, salt, spiders, etc., accumulate in the vent line and/or an air filter positioned in the vent line. If the vent line is blocked, fresh air cannot be drawn on the fuel vapor canister. The canister cannot be purged, yet fuel vapor will continue to be adsorbed within the canister until the adsorbent is saturated. This will lead to an increase in bleed emissions.
The inventors herein have recognized the above problems, and have developed systems and methods to at least partially address them. In one example, a method for purging a fuel vapor canister buffer, comprising: opening a fuel tank isolation valve; opening a canister purge valve; and drawing a vacuum on a fuel tank sufficient to open a capless refueling assembly vacuum relief mechanism. In this way, the fuel vapor canister buffer may still be purged to intake even under conditions where the canister vent line is blocked.
In another example, a fuel system for a vehicle, comprising: a fuel tank coupled to a buffer of a fuel vapor canister; a capless refueling assembly coupled to the fuel tank, the capless refueling assembly configured to vent to atmosphere responsive to a fuel tank vacuum increasing above a threshold vacuum; an engine intake coupled to the fuel vapor canister; and a controller configured with instructions stored in non-transitory memory, that when executed, cause the controller to: during a first condition, apply a vacuum from the engine intake to the fuel tank such that the capless refueling assembly vents to atmosphere; and maintain applying vacuum from the engine intake to the fuel tank until a load of the buffer of the fuel vapor canister decreases below a threshold. In this way, the canister may be partially purged to intake. Following a diurnal cycle, the fuel vapor remaining in the canister may migrate into the canister buffer. The cycle may then be repeated. In this way, the contents of the canister may be gradually purged to intake, decreasing bleed emissions that would otherwise occur if the vent line is blocked.
In yet another example, a method for purging a fuel vapor canister, comprising: during a first condition, opening a fuel tank isolation valve; ramping up a canister purge valve duty cycle until a capless refueling assembly vents to atmosphere; drawing atmospheric air into the engine intake via a path that includes the capless refueling assembly, the fuel tank, and the fuel vapor canister buffer; drawing fuel vapor desorbed from the fuel vapor canister buffer into the engine intake; maintaining the canister purge valve duty cycle until an exhaust gas oxygen sensor indicates a richness of exhaust has decreased below a threshold; and then closing the fuel tank isolation valve and canister purge valve. In this way, a secondary canister vent pathway may be realized without adding any additional hardware and thus without increasing manufacturing costs.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.